Sulphur dioxide is a typical by-product of industrial processes such as the smelting of sulphide ores or burning of sulphur containing fuels. If vented to the atmosphere without treatment, it can cause environmental problems such as acid rain.
The treatment of sulphur dioxide by thermal reduction to sulphur using methane, or a gas containing predominantly methane, such as natural gas, is known. However, in practice, such thermal reduction processes suffer from either soot formation or the need to add steam to the reaction mixture to avoid soot formation. In both scenarios, particularly the second, fuel consumption is high.
UK 1213760.0 discloses a process for producing sulphur from sulphur dioxide using thermal reduction with natural gas which has been successfully demonstrated on an industrial scale unit.
This process operates with high sulphur recovery efficiency and is efficient in the overall use of natural gas if a high temperature incinerator is needed downstream of the process. Such a high temperature incinerator can use the fuel values in the process gas as fuel.
Alternative processes have been proposed which use a hydrogenation catalyst following the thermal stage in order to react the reducing gases with sulphur dioxide.
The disadvantage of these processes are numerous and include:
i) the hydrogenation of sulphur which reduces the overall sulphur recovery efficiency,
ii) the high heat of reaction of the hydrogenation processes leads to high reactor outlet temperatures and a further loss of sulphur recovery efficiency,
iii) some hydrogenation catalysts have a tendency to form carbonyl sulphide, which also leads to a reduction in the sulphur recovery efficiency,
iv) the catalysts typically contain iron, nickel or cobalt in sulphide form, which can lead to high sulphur dioxide emission during plant shutdown operations, and environmental issues for spent catalyst disposal,
v) the catalysts are vulnerable to damage because the oxide form can be reduced to the metallic form during typical plant start-up and standby operations when natural gas and air are fired sub-stoichiometrically in the thermal reactor,
vi) substantially no hydrogen remains in the tail gases from the Claus stages with the consequence that hydrogen needed for tail gas clean-up processes, such as the SCOT process, must be provided separately, and
vii) substantially no fuel values remain in the tail gases for the Claus stages, so that all the fuel needed for an incinerator must be supplied separately.
U.S. Pat. No. 5,795,554, GB2087373 and U.S. Pat. No. 5,795,554 disclose catalysts for reacting carbon monoxide and sulphur dioxide to form carbon dioxide and sulphur; however, they do not show titanium dioxide to be a useful catalyst for the above reaction.
The present invention addresses these and other problems with prior art processes.